Hurwitz's Automorphisms Theorem - Interpretation in Terms of Hyperbolicity

Interpretation in Terms of Hyperbolicity

One of the fundamental themes in differential geometry is a trichotomy between the Riemannian manifolds of positive, zero, and negative curvature K. It manifests itself in many diverse situations and on several levels. In the context of compact Riemann surfaces X, via the Riemann uniformization theorem, this can be seen as a distinction between the surfaces of different topologies:

• X a sphere, a compact Riemann surface of genus zero with K > 0;
• X a flat torus, or an elliptic curve, a Riemann surface of genus one with K = 0;
• and X a hyperbolic surface, which has genus greater than one and K < 0.

While in the first two cases the surface X admits infinitely many conformal automorphisms (in fact, the conformal automorphism group is a complex Lie group of dimension three for a sphere and of dimension one for a torus), a hyperbolic Riemann surface only admits a discrete set of automorphisms. Hurwitz's theorem claims that in fact more is true: it provides a uniform bound on the order of the automorphism group as a function of the genus and characterizes those Riemann surfaces for which the bound is sharp.